Compact and Efficient Heat Exchanger, Furnace, HVAC Unit, Building, and Method of Making

ABSTRACT

Compact and efficient apparatuses for transferring heat, heat exchangers, and furnaces, as well as HVAC units, HVAC systems, and buildings therewith, plus methods of making such devices. Furnace heat exchangers, for example, have an added stage, added in an unconventional way, that improves efficiency without unduly increasing size, while maintaining required radiuses of bends in tubing. Various embodiments orient bends at particular angles to allow stages to fit together more closely, provide improved structure for connecting stages using two collectors that seal against a common junction plate, provide for indoor air passing through the heat exchanger to pass through different passes of the heat exchanger in an unconventional order in order to provide greater compactness, or a combination thereof, as examples.

FIELD OF THE INVENTION

This Invention relates to apparatuses for transferring heat, heat exchangers, furnaces, heating, ventilation, and air conditioning (HVAC) units, HVAC systems, buildings having such devices, and methods of manufacturing such devices.

BACKGROUND OF THE INVENTION

Apparatuses for transferring heat, such as heat exchangers, have been used in the past to transfer heat from one fluid (e.g., liquid or gas) to another. In furnaces, for example, one or more fuels, such as natural gas, propane, liquefied petroleum gas (LPG, LP, or GLP), butane, methane, heating oil, gasoline, alcohol, coal, wood, or biomass, has been burned and one or more heat exchangers having one or more stages has been used to transfer heat from the products of combustion to indoor air (e.g., air inside or delivered to a building or other occupied space), for instance. Further, furnaces have been incorporated into or formed heating, ventilation, and air conditioning units, various embodiments of which have included one or more fans, and some of which have included direct expansion air conditioning systems, for example. Furnaces and HVAC units have been used, for example, to change or control the temperature within buildings to provide a comfortable and safe environment for people to live or work, for example.

Counter-flow heat exchanges are known in the art, and furnaces have been made with multiple stages, some of which have had multiple passes. Typical furnaces have had a first stage having three passes, joined to a second stage that had a single pass and that had fins to enhance heat transfer. The efficiency of such furnaces was limited to about 90% (AFUE), but notions of adding further stages were rejected in order to keep heat exchangers and furnaces compact and inexpensive.

Thus, needs or potential for benefit exist for apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that are more efficient, and yet are more compact than prior art alternatives having comparable efficiency or cost. Needs or potential for benefit also exist for such apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that have connections between stages that provide for compactness and yet are reliable and cost competitive. Needs or potential for benefit also exist for methods of manufacturing or making such apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that are conducive to mass production, cost effective, and reliable. Room for improvement exists over prior art in these and other areas that may be apparent to a person of ordinary skill in the art having studied this document.

SUMMARY OF PARTICULAR EMBODIMENTS OF THE INVENTION

This invention provides, among other things, various apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, and methods of manufacturing or making such devices. Various embodiments provide, as objects or benefits, for example, that they are more efficient, more compact, less expensive, or a combination thereof, in comparison with various alternatives. Further, some embodiments are reliable, have short manufacturing times, produce high quality units, or a combination thereof. Other benefits of certain embodiments may be apparent to a person of ordinary skill in the art. Other embodiments of the invention, include various HVAC systems and buildings having such apparatuses for transferring heat, heat exchangers, furnaces, or HVAC units, as further examples.

In specific embodiments, this invention provides various apparatuses for transferring heat or heat exchangers, for example. In various embodiments, the apparatus may be, or may be part of, a furnace, an HVAC unit, an HVAC system, or a building that has an HVAC system, as examples. In a number of embodiments, such an apparatus may include, for example, a first heat-exchanger stage and a second heat-exchanger stage. In certain embodiments, the first heat-exchanger stage may include, for example, multiple parallel first-stage tubes. And in some embodiments, each first-stage tube may have a first 180 degree bend and a second 180 degree bend, for example. Further, in particular embodiments, the second heat-exchanger stage may include, for example, multiple parallel second-stage tubes. In some embodiments, each second-stage tube may have a third 180 degree bend, for instance.

In some such embodiments, the first-stage tubes and the second-stage tubes may be configured to contain within a flowing first fluid, for instance, and may be configured to transfer heat between the first fluid and a second fluid external to the first-stage tubes and to the second-stage tubes, for example. Further, in some such embodiments, when the apparatus, unit, or system is in operation, for instance, the second fluid may flow in a predominant flow direction past the first-stage tubes and past the second-stage tubes, for example. In some embodiments, this predominant flow direction may be up, for example.

Further, in a number of embodiments, the first 180 degree bend may be oriented at a first angle from the predominant flow direction, and in some such embodiments the absolute value of the first angle may be between 15 degrees and 75 degrees, for example. Moreover, in particular embodiments, the second 180 degree bend may be oriented at a second angle from the predominant flow direction, and in some such embodiments the absolute value of the second angle may be between 15 degrees and 75 degrees, for instance. Furthermore, in certain embodiments, the third 180 degree bend may be oriented at a third angle from the predominant flow direction, and in some such embodiments, the absolute value of the third angle may be less than 15 degrees, for example. In some such embodiments, the first angle and the second angle have opposite signs, for instance.

In other embodiments, the absolute value of the first angle may be between 30 degrees and 60 degrees, the absolute value of the second angle may be between 30 degrees and 60 degrees, the absolute value of the third angle may be less than 10 degrees, or a combination thereof. Further, in some embodiments, the absolute value of the first angle may be between 40 degrees and 50 degrees, the absolute value of the second angle may be between 40 degrees and 50 degrees, the absolute value of the third angle may be less than 5 degrees, or a combination thereof, as other examples.

In some embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage and then through the second heat-exchanger stage, for example. And in some embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged in an order such that the second fluid passes first through one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage, for instance. Further, in various embodiments, the first-stage tubes have a first diameter and the second-stage tubes have a second diameter. In some embodiments, the first diameter may be substantially larger than the second diameter, for example.

The apparatus or unit may further include, in various embodiments, a junction plate, and in some such embodiments, one end of each of the first-stage tubes may terminate at the junction plate, at least one end of each of the second-stage tubes may terminate at the junction plate, or a combination thereof, for example. Further, in some embodiments, the apparatus or unit may further include, for instance, a first collector that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes. The first enclosed passageway may transfer the first fluid (e.g., combustion gasses) from the first-stage tubes to the second-stage tubes, for example.

Certain embodiments may include, for example, at least one, or even multiple burners. In some such embodiments, for instance, each of the first-stage tubes has an entrance end where air enters the first-stage tube, and a burner may be located at the entrance end to one or more (e.g., each) of the first-stage tubes. In a number of embodiments, each burner may be configured to burn a fuel (e.g., in one of the first-stage tubes), for example, forming combustion gasses. In some embodiments, for instance, combustion gasses from the air and the fuel specifically form the first fluid.

Some such apparatuses or units may further include, for example, a third heat-exchanger stage which may include, for instance, multiple parallel third-stage tubes. In some of these embodiments, the third heat-exchanger stage may include, for instance, multiple fins, which may be external to the third-stage tubes, for example. In specific embodiments, the first heat-exchanger stage has three passes through the second fluid, the second heat-exchanger stage has two passes through the second fluid, the third heat-exchanger stage has one pass through the second fluid, or a combination thereof, for example.

Moreover, in some embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged in an order such that the second fluid passes first through the third heat-exchanger stage, and then through at least one pass of the second heat-exchanger stage, and such that the second fluid passes through at least one pass of the first heat-exchanger stage last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage. Furthermore, in some embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged in an order such that the second fluid passes first through the third heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage. In various embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage.

Further, in a number of embodiments, the first-stage tubes have a first diameter, the second-stage tubes have a second diameter, and the third-stage tubes have a third diameter. In addition, in various embodiments, the first diameter may be substantially larger than the second diameter, the second diameter may be substantially larger than the third diameter, or both, for instance. In addition, or instead, in some embodiments, the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, and the third heat-exchanger stage has a third number of tubes. Furthermore, in some embodiments, the first number of tubes may be equal to or one less than the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, for example.

Further still, in certain embodiments, one end of each of the first-stage tubes terminates at the junction plate, two ends of each of the second-stage tubes terminate at the junction plate, one end of each of the third-stage tubes terminates at the junction plate, or a combination thereof, for example. In some embodiments, the apparatus may further include, for instance, the first collector (e.g., that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes), a second collector that seals against the junction plate and forms a second enclosed passageway that connects the second-stage tubes to the third-stage tubes, or both the first collector and the second collector, for example. The second enclosed passageway may transfer the first fluid (e.g., combustion gasses) from the second-stage tubes to the third-stage tubes, for example.

This invention also provides various methods, including methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel. Such methods may include, for example, (e.g., in any order, unless a particular order is required or indicated) at least certain acts. In some embodiments, such acts may include, for example, an act of forming or obtaining first-stage tubes. In particular embodiments, each first-stage tube may have a first diameter, a first 180 degree bend, a second 180 degree bend, or a combination thereof, for example. Another act found in various embodiments is an act of forming or obtaining second-stage tubes. In certain embodiments, each second-stage tube may have a second diameter, a third 180 degree bend, or both, for instance.

Some methods also include an act of assembling a first heat-exchanger stage, for instance, using multiple of the first-stage tubes. In some such embodiments, the first heat-exchanger stage may include, for instance, multiple passes, such as a first pass, a second pass, and a third pass. Another act in various methods may be an act of assembling a second heat-exchanger stage, for instance, using multiple of the second-stage tubes. In some such embodiments the second heat-exchanger stage may include, for instance, one or more passes, such as a fourth pass and a fifth pass, for example. Further, some methods include an act of installing a burner, for example, at an entrance end of (e.g., each) first-stage tube. And some embodiments include an act of connecting the second heat-exchanger stage to the first heat-exchanger stage, for instance, so that, when the furnace is in operation, products of combustion from the burning of the fuel (e.g., at the burner) pass first through the first heat-exchanger stage and then through the second heat-exchanger stage, for example, passing first through the first-stage tubes and then through the second-stage tubes.

In some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage, and assembling the second heat-exchanger stage, for example, (or other acts) may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage, for example, so that indoor air passing through the furnace (e.g., when the furnace is in operation), passes first through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass, for example.

In various embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage, and assembling the second heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage so that (e.g., when the furnace or unit is in operation), the indoor air flows in a predominant flow direction, for example, past the first-stage tubes, past the second-stage tubes, or both. In some such embodiments, the first 180 degree bend may be oriented at a first angle (e.g., from the predominant flow direction), and the absolute value of the first angle may be between 15 degrees and 75 degrees, for example. Similarly, in some embodiments, the second 180 degree bend may be oriented at a second angle (e.g., from the predominant flow direction), and the absolute value of the second angle may be between 15 degrees and 75 degrees, for instance. Further, in some embodiments, the third 180 degree bend may be oriented at a third angle (e.g., from the predominant flow direction), and the absolute value of the third angle may be less than 15 degrees, for example. In other embodiments, some or all of these angles may fall within other ranges described herein, as examples. Moreover, in a number of embodiments, various acts, such as the acts of forming or obtaining first-stage tubes and assembling the first heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and assembling and arranging the first heat-exchanger stage so that the first angle and the second angle have opposite signs.

Certain embodiments may further include, for example, an act of forming or obtaining a third heat-exchanger stage, which may have multiple third-stage tubes, for example. In some such embodiments the third heat-exchanger stage may include, for instances a sixth pass. Further, in some embodiments, the act of forming or obtaining a third heat-exchanger stage may further include, or another act in a method may be or include, for instance, forming or obtaining a third heat-exchanger stage which may have multiple fins external to the third-stage tubes.

Various methods may further include, for example, an act of connecting the third heat-exchanger stage to the second heat-exchanger stage, for instance, so that (e.g., when the furnace or unit is in operation), products of combustion from burning of the fuel (e.g., at the burner) pass first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage, for instance, passing first through the first-stage tubes, then through the second-stage tubes, and then through the third-stage tubes. Further, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, forming or obtaining the third heat-exchanger stage, assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage, for example, may further include, (or one or more other acts may include) for instance, assembling and arranging the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage so that indoor air passing through the furnace, for example, (e.g., when the furnace or unit is in operation) specifically passes first through the sixth pass, then through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass, for example.

Additionally, various embodiments of methods may include, for example, (e.g., in any order) the acts of forming or obtaining a junction plate, forming or obtaining a first collector, forming or obtaining second collector, or a combination thereof, for instance. Further, in some embodiments, various acts, such as the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage, for example, may further include, for instance, (e.g., in any order) the acts of terminating one end of each of the first-stage tubes at the junction plate, terminating two ends of each of the second-stage tubes at the junction plate, terminating one end of each of the third-stage tubes at the junction plate, or a combination thereof, for instance. Some such embodiments may further include one or both acts of installing the first collector against the junction plate forming a first enclosed passageway, for example, that connects the first-stage tubes to the second-stage tubes, or installing the second collector against the junction plate forming a second enclosed passageway, for example, that connects the second-stage tubes to the third-stage tubes.

Furthermore, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, and forming or obtaining the third heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes and forming or obtaining the third heat-exchanger stage so that the third-stage tubes have a third diameter. In some embodiments, the first diameter may be substantially larger than the second diameter, the second diameter may be substantially larger than the third diameter, or both, as examples.

Moreover, in some embodiments, particular acts, such as the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage include, for instance, assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage so that the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, the third heat-exchanger stage has a third number of tubes. In certain embodiments, the first number of tubes may be equal to or one less than the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, as examples. In addition, various other embodiments of the invention are also described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view illustrating, among other things, an example of an apparatus for transferring heat, which is an example of a heat exchanger, and which may be used in a furnace or an HVAC unit, as examples;

FIG. 2 is a side view illustrating a building having an HVAC system and an HVAC unit, which may include the apparatus of claim 1, for example;

FIG. 3 is an isometric view of the apparatus for transferring heat of FIG. 1, from a different angle, showing, among other things, the fan that moves air and products of combustion through the apparatus;

FIG. 4 is a side view of the apparatus for transferring heat of FIGS. 1 and 3, showing, among other things, the junction plate;

FIG. 5 is the same side view as FIG. 4, of the apparatus for transferring heat of FIGS. 1 and 3, except with the junction plate removed, showing, among other things, the orientation angles of various 180 degree bends in the tubing, in the embodiment depicted, and the order of different passes of the heat exchanger;

FIG. 6 is a partial side view, taken from the same angle as FIG. 5, also with the junction plate removed, but showing the 180 degree bends of the second heat-exchanger stage oriented at a slightly different angle; and

FIG. 7 is a flow chart illustrating an example of a method of manufacturing a heat exchanger or a furnace for heating indoor air by burning a fuel, for example.

The drawings illustrate, among other things, various particular examples of embodiments of the invention, and certain examples of characteristics thereof. Different embodiments of the invention include various combinations of elements or acts shown in the drawings, described herein, known in the art, or a combination thereof, for example.

DETAILED DESCRIPTION OF EXAMPLES OF EMBODIMENTS

Among other things, various embodiments of the invention are, or include, apparatuses for transferring heat, or heat exchangers, for example. In various embodiments, the apparatus may be, or may be part of, a furnace, an HVAC unit, an HVAC system, or a building that has an HVAC system, as examples. Further, a number of embodiments are, or include, methods, including methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel.

In various embodiments, examples of which are described in detail below, a second heat-exchanger stage may be added to an apparatus for transferring heat or a heat exchanger (e.g., in comparison with a typical prior art apparatus or heat exchanger). In different embodiments, this second heat-exchanger stage may be added in an unconventional way or configuration, examples of which are described herein, that improves efficiency without unduly increasing the size of the apparatus, while maintaining required radiuses of bends in heat exchanger tubing. In some embodiments, the second heat-exchanger stage is connected to the other stages in a novel way, for another example.

As described herein, various embodiments orient bends (e.g., 180 degree bends in heat-exchanger tubing) at particular angles to allow heat-exchanger stages to fit together more closely. Some embodiments provide improved structure for connecting stages, for example, using two collectors that seal against a common junction plate. Further, some embodiments provide for indoor air passing through the heat exchanger or apparatus to pass through different passes of the heat exchanger, apparatus, or stages, in a particular unconventional order in order to allow for surprisingly greater compactness. Some embodiments further restrict or combine such features, as described herein.

Specifically, FIGS. 1 and 3 illustrate an example of an apparatus for transferring heat or a heat exchanger, apparatus 100. In various embodiments, apparatus 100 may be part of a furnace, a heating, ventilation, and air conditioning unit (an HVAC unit), an HVAC system, or a building that has an HVAC system, as examples. For example, FIG. 2 illustrates building 200 with HVAC unit 250 mounted on roof 202 of building 200. HVAC unit 250, along with supply air ductwork 275, supply air registers 271, 272, and 273, thermostat or controller 274, return air grille 278, air filter 279, and return air ductwork 277, among other components, form HVAC system 270.

Building 200, in the embodiment illustrated, includes, besides HVAC system 270, roof 202, ceiling 203, floor 201, and walls 204, among other components. In the embodiment shown, walls 204, ceiling 203, and floor 201 form enclosure 215 enclosing space 220, which contains indoor air 210. In the embodiment illustrated, indoor air 210 is drawn through return air grille 278, through filter 279 and return air ductwork 277 to HVAC unit 250 by fan 255 (which is a component of HVAC unit 250). Indoor air 210 then passes through apparatus or heat exchanger 100 where indoor air 210 is heated, for example, and then indoor air 210 is blown (e.g., by fan 255) through supply air ductwork 275 and registers 271, 272, and 273 back to space 220 within enclosure 215. Fan 255 is shown in FIG. 2 in one example of a location within HVAC unit 250. In other embodiments, fan 255 may be located below heat exchanger 100 and may blow indoor air 210 upwards (e.g., in direction 110 shown in FIGS. 1 and 3) through heat exchanger 100, as another example.

In the embodiment illustrated, apparatus or heat exchanger 100 may be used to heat indoor air 210 when conditions warrant. In some embodiments, HVAC unit 250 may also include an air conditioning system, such as direct expansion air conditioning, for cooling indoor air 210 when conditions require cooling of space 220. Thus, in various embodiments, air conditioning unit 250 may include a compressor, evaporator coil, condenser coil, expansion valve, condenser fan, etc. In the embodiment illustrated, HVAC unit 250 is a packaged unit, but in other embodiments, split system air conditioning systems may be used, as another example. Various fans may be driven by electric motors for example. In other embodiments, HVAC unit 250 may be (just) a furnace, and may lack an air conditioning system, for example, for climates or applications where cooling is not required. As used herein, the phrases “HVAC unit” and “HVAC system” are general terms and include units and systems that do not have air conditioning. As used herein, a stand-alone furnace, having a ventilation fan, is considered to be an HVAC unit. A furnace may also be part of an HVAC unit that further includes air conditioning, as such terms are used herein.

Returning to FIGS. 1 and 3, and also referring to FIGS. 4 and 5, in the embodiment illustrated, apparatus 100 includes, for example, first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123. Some embodiments require, or involve improvements to, just first heat-exchanger stage 121 and second heat-exchanger stage 122, for example. In the embodiment shown, first heat-exchanger stage 121 includes, for example, five parallel first-stage tubes 131. Other embodiments may have a different number of first-stage tubes 131, such as 1, 2, 3, 4, 6, 7, 8, 9, 10, 12, 15, or 20 first-stage tubes 131, as examples, which (e.g., if multiple first-stage tubes are used) may be positioned in parallel as shown or in one or more rows.

In different embodiments, tubes (e.g., 131) of a heat-exchanger stage (e.g., 121) may be geometrically parallel (e.g., to within a manufacturing tolerance), may be parallel with respect to the flow of a fluid [e.g., the first fluid or combustion products contained within the tubes (e.g., 131), as described below], or both. As used herein, unless apparent otherwise, “parallel” means either geometrically parallel (e.g., to within a manufacturing tolerance), parallel with respect to the flow of a fluid (e.g., the first fluid or combustion products, as described below), or both. Further, as used herein, “fluids” may be liquids, gasses, or a mixture thereof, in various embodiments.

In the embodiment illustrated, each first-stage tube 131 has a first 180 degree bend 141 and a second 180 degree bend 142. Further, in the embodiment shown, the second heat-exchanger stage 122 includes five parallel second-stage tubes 132. Other embodiments may have a different number of second-stage tubes 132, such as 1, 2, 3, 4, 6, 7, 8, 9, 10, 12, 13, 15, 16, 20, or 21 second-stage tubes 132, as examples, which may be positioned in parallel as shown (e.g., in one or more rows). In this embodiment, each second-stage tube 132 has a third 180 degree bend 143. As used herein, 180 degree bends may vary slightly from an exact 180 degrees, but may need to meet required tolerances. In some embodiments, for example, tolerances for angle of bends, parallelness of tubing, orientation of angles, diameters, lengths, etc., may be consistent with other tolerances employed in similar prior art equipment, for instance.

In the embodiment illustrated, first-stage tubes 131 and second-stage tubes 132 are configured to contain within a flowing first fluid, and are configured to transfer heat between the first fluid and a second fluid (e.g., indoor air 210) external to first-stage tubes 131 and to second-stage tubes 132. Further, in this embodiment, when apparatus 100, HVAC unit 250, or HVAC system 270, for example, is in operation, the second fluid (e.g., indoor air 210) flows in a predominant flow direction 110 past first-stage tubes 131 and past second-stage tubes 122, for example. In this embodiment, the predominant flow direction 110 is upwards (up). In other embodiments, however, the predominant flow direction (e.g., 110) may be in another direction, such as downwards, horizontal, or at an angle, as examples.

Referring to FIG. 5, in a number of embodiments, first 180 degree bend 141 (e.g., of each first-stage tube 131) may be oriented at a first angle 501 from the predominant flow direction 110. As used herein, the orientation of a 180 degree bend is measured from the perspective of 180 degree bends 141, 142, or 143 shown in FIG. 5, for example. Further, as used herein, the orientation of a 180 degree angle, relative to the predominant flow direction 110, for example, is, considered to be a positive angle (i.e., have a positive sign) if the orientation angle (less than 90 degrees) is to the right, and the orientation of a 180 degree angle, relative to the predominant flow direction 110, for example, is, considered to be a negative angle (i.e., have a negative sign) if the orientation angle (less than 90 degrees) is to the left. Thus, first angle 501 of first 180 degree bend 141 (e.g., of each first-stage tube 131) has a positive angle (e.g., a positive sign).

In the embodiment illustrated, first angle 501 is about 45 degrees, specifically, and is positive. In various other embodiments, first angle 501 may have another angle, may be negative, or both, for instance. For example, in a number of embodiments, the absolute value of first angle 501 may be between 15 degrees and 75 degrees. In fact, in some embodiments, the absolute value of first angle 501 may be between 30 degrees and 60 degrees, and in particular embodiments, the absolute value of first angle 501 may be between 40 degrees and 50 degrees, as examples.

Additionally, in the embodiment illustrated, second 180 degree bend 142 (e.g., of each first-stage tube 131) is oriented at second angle 502 from the predominant flow direction. In the embodiment illustrated, second angle 502 is about 45 degrees, specifically, and is negative. In various other embodiments, second angle 502 may have another angle, may be positive, or both, for instance. For example, similar to first angle 501, in a number of embodiments, the absolute value of second angle 502 may be between 15 degrees and 75 degrees. In fact, in some embodiments, the absolute value of second angle 502 may be between 30 degrees and 60 degrees, and in particular embodiments, the absolute value of second angle 502 may be between 40 degrees and 50 degrees, as examples.

FIG. 6 illustrates an alternate embodiment to second heat-exchanger stage 122, namely, second heat-exchanger stage 622. In this embodiment, third 180 degree bend 143 (e.g., of each second-stage tube 132) is oriented at third angle 603 from the predominant flow direction 110. In the embodiment illustrated, third angle 603 is about 30 degrees, specifically, and is negative. In various other embodiments, third angle 603 may have another angle, may be positive, or both, for instance. For example, in a number of embodiments, the absolute value of third angle 603 may be less than 15 degrees. In fact, in some embodiments, the absolute value of third angle 603 may be less than 10 degrees, or even less than 5 degrees, as examples. In the embodiment shown in FIGS. 1 and 3-5, for instance, the third angle (e.g., corresponding to angle 603) is about zero degrees, as another example.

Certain embodiments may include, for example, at least one, or even multiple burners. In the embodiment shown in FIGS. 1 and 3-5, for instance, each of the first-stage tubes 131 has an entrance end 151 (e.g., shown in FIG. 1) where air enters the first-stage tube 131, and a burner 155 (e.g., shown in FIGS. 1 and 3) is located at the entrance end 151 to each of the first-stage tubes 131. In a number of embodiments, each burner (e.g., 155) may be configured to burn a fuel, for example, in one of the first-stage tubes 131. Such a fuel may be natural gas, propane, LPG, butane, or heating oil, as examples. In various embodiments, the burning of the fuel may form flue products or combustion gasses. In many embodiments, for instance, flue products or combustion gasses from the air and the fuel specifically form the first fluid referred to herein.

As shown in FIGS. 1, 3, and 4, in the embodiment shown, apparatus 100 further includes junction plate 160. In this embodiment, one end 351 (e.g., shown in FIG. 3) of each of the first-stage tubes 131 terminates at junction plate 160. In addition, in this embodiment, both (two) ends 352 of each of the second-stage tubes 132 terminate at junction plate 160. Further, as shown in FIG. 1, in this particular embodiment, apparatus 100 further includes first collector 161. First collector 161, in this embodiment, seals against junction plate 160 forming a first enclosed passageway that connects first-stage tubes 131 to second-stage tubes 132. The first enclosed passageway (e.g., formed between junction plate 160 and first collector 161) transfers the first fluid (e.g., combustion gasses) from the first-stage tubes 131 to the second-stage tubes 132, in this embodiment. Thus, in the embodiment shown in FIGS. 1 and 3-5, first heat-exchanger stage 121 and second heat-exchanger stage 122 are arranged and connected in an order such that the first fluid or combustion gasses pass first through first heat-exchanger stage 121 and then through second heat-exchanger stage 122.

Further still, in the embodiment shown, one end of each of the third-stage tubes 133 terminates at junction plate 160. In this embodiment, apparatus 100 further includes second collector 162 that seals against junction plate 160 and forms a second enclosed passageway that connects second-stage tubes 132 to third-stage tubes 133. The second enclosed passageway, in this embodiment, transfers the first fluid (e.g., combustion gasses) from second heat-exchanger stage 122 to third heat-exchanger stage 123. The first collector 161 and the second collector 162 may be located in or attached to the same (e.g., vertical) plane of junction plate 160, for example, as shown. The first collector 161 and the second collector 162 may be attached to junction plate 160 with fasteners, in some embodiments, such as sheet metal screws, for instance.

Moreover, in the embodiment illustrated, first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123 are arranged and connected in an order such that the first fluid (e.g., combustion gasses) passes first through first heat-exchanger stage 121, then through second heat-exchanger stage 122, and then through third heat-exchanger stage 123. Specifically, in the embodiment illustrated, fan 355 (shown in FIG. 3) draws air or the first fluid (e.g., combustion gasses) from third heat-exchanger stage 123, which draws (through connector 162) from second heat-exchanger stage 122, which draws (through connector 161) from first heat-exchanger stage 121, which draws air or the first fluid (e.g., combustion gasses) through burners 155. In a number of embodiments, fan 355 may exhaust the first fluid or products of combustion outside of HVAC unit 250 and outside of building 200, for example.

In a number of embodiments, since fan 355 draws the combustion gasses through apparatus 100 (e.g., rather than blowing air into burners 155), the pressure of the combustion gasses is lower than atmospheric pressure. In addition, in many embodiments, the pressure of the second fluid (e.g., indoor air 210) within apparatus 100 may be kept above atmospheric pressure (e.g., by blowing indoor air 210 into apparatus 100 with fan 255). Thus, if a leak develops, for example, within first heat-exchanger stage 121, second heat-exchanger stage 122, third heat-exchanger stage 123, or connections (e.g., collectors 161 or 162) therebetween, the products of combustion do not leak into space 220 threatening occupants thereof, for example, with asphyxiation.

Referring to FIG. 4, in the embodiment illustrated, apparatus 100 includes third heat-exchanger stage 123, which includes multiple parallel third-stage tubes 133, which are arranged in two rows 471 and 472. In the embodiment depicted, the third heat-exchanger stage 123 includes multiple fins 333 (e.g., shown in FIG. 3), which are external to third-stage tubes 133. In other embodiments, a third heat-exchanger stage may have third-stage tubes formed into 1, 3, 4, 5, 6, 7, 8, or 10 rows, as other examples. Further, in some embodiments, the third heat-exchanger stage may omit fins.

In some embodiments, some or all of the stages (e.g., first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123) may have multiple passes (e.g., through the second fluid or indoor air 210). Referring to FIGS. 1 and 5, in the embodiment illustrated, for example, first heat-exchanger stage 212 has three passes 571, 572, and 573 (e.g., the first, second, and third passes) through the second fluid, the second heat-exchanger stage 122 has two passes 574 and 575 (e.g., the fourth and fifth passes) through the second fluid, and the third heat-exchanger stage 123 has one pass 576 (e.g., the sixth pass) through the second fluid. In different embodiments, some or all of the stages (e.g., first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123) may have 1, 2, 3, 4, 5, 6, or 7 passes (e.g., through the second fluid or indoor air 210), for example, in various combinations.

Furthermore, in the embodiment illustrated, the first heat-exchanger stage 121 and the second heat-exchanger stage 122 are arranged in an order such that the second fluid (e.g., indoor air 210) passes (e.g., flowing in the predominant airflow direction 110, for example, up) first through one pass (e.g., pass 575) of second heat-exchanger stage 122, then through one pass (e.g., pass 573) of first heat-exchanger stage 121, then through one pass (e.g., pass 574) of second heat-exchanger stage 122, and then through two passes (e.g., 572 and then 571) of first heat-exchanger stage 121. In various other embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged in an order such that the second fluid passes first through one or more passes of the second heat-exchanger stage, then through one or more passes of the first heat-exchanger stage, then through one or more passes of the second heat-exchanger stage, and then through one or more passes of the first heat-exchanger stage, for instance.

Such unconventional configurations or departures from typical multi-pass counter-flow heat exchangers, for example, may allow the first and second heat-exchanger stages (e.g., 121 and 122 respectively) to be packed more closely together, for example, with a given radius of bend of the different 180 degree bends (e.g., 141, 142, and 143). This may allow the apparatus or heat exchanger (e.g., 100) to be more compact, for example. The radius of the 180 degree bends (e.g., 141, 142, and 143) may be limited (e.g., to a minimum radius) based on the diameter of the tubing, wall thickness, material used, internal flow resistance that is acceptable, structural considerations, etc. In some cases, tighter bends may require the use of fittings, which may increase cost, provide more potential for leakage, increase internal flow resistance (and therefore fan power, for example, for fan 355), require more structural components, or the like, as examples.

In addition, in some embodiments, this unconventional configuration may surprisingly allow more stages (e.g., three stages instead of two) to be packed within the available space for the apparatus or heat exchanger (e.g., 100), thus, allowing for a greater heat transfer efficiency. In the embodiment of apparatus 100, for example, heat exchanger efficiency increased from 90% to 95% Annual Fuel Utilization Efficiency (AFUE) by adding second heat-exchanger stage 122, and reached a steady state efficiency of 99%. Other embodiments may provide other levels of performance. Improvements in heat transfer efficiency reduce the amount of fuel that must be burned, thereby reducing fuel bills, reducing the emission of greenhouse gasses, and reducing dependency on sources of fossil fuels, among other potential benefits.

Further, in the embodiment illustrated, first-stage tubes 131 have a first diameter and second-stage tubes 132 have a second diameter. In the embodiment illustrated, the first diameter is substantially larger than the second diameter, for example. As used herein, substantially larger means larger by more than ten percent (10%). Specifically, in some embodiments, the first diameter (e.g., of first-stage tubes 131) is 1.75 inches, and the second diameter (e.g., of second-stage tubes 132) is 1.0 inches, for example. In other embodiments, the first diameter (e.g., of first-stage tubes 131) may be 1.25, 1.5, 1.625, 1.875, 2.0, 2.25, or 2.5 inches, or 3.5, 4.0, 4.5, 5.0 5.5, or 6.0 cm, as examples, and the second diameter (e.g., of second-stage tubes 132) may be 0.75, 0.875, 1.125, 1.25, or 1.5 inches, or 2.0, 2.25, 2.5, 2.75, or 3.0 cm, for example.

In some embodiments, first-stage tubes 131, second-stage tubes 132, plate 160, covers 161 and 162, or a combination thereof, may be made of aluminized steel, or stainless steel, as examples. In some embodiments, the third heat-exchanger stage 123 may be made of stainless steel, for example, 29-4C stainless steel. Further, in a number of embodiments, third-stage tubes 133 have a third diameter, and, in various embodiments, the second diameter (e.g., of second stage tubes 132) may be substantially larger than the third diameter, for instance. In the embodiment illustrated, the second diameter of second-stage tubes 132 is shown to be substantially larger than the third diameter of the third-stage tubes 133.

Moreover, in some embodiments, the first heat-exchanger stage (e.g., 121), the second heat-exchanger stage (e.g., 122), and the third heat-exchanger stage (e.g., 123) may be arranged in an order such that the second fluid (e.g., indoor air 210) passes first through the third heat-exchanger stage (e.g., 123), and then through at least one pass of the second heat-exchanger stage, (e.g., 122) and such that the second fluid passes through at least one pass of the first heat-exchanger stage (e.g., 121) last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage. In the specific embodiment illustrated, for example, first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123 are arranged in apparatus 100 in an order such that the second fluid (e.g., indoor air 210) passes first through pass 576 of third heat-exchanger stage 123, and then through pass 575 of second heat-exchanger stage 122, and such that the second fluid passes through pass 571 of first heat-exchanger stage 121 last of first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123.

Furthermore, in some embodiments, the first heat-exchanger stage (e.g., 121), the second heat-exchanger stage (e.g., 122), and the third heat-exchanger stage (e.g., 123) may be specifically arranged in an order such that the second fluid passes first through the third heat-exchanger stage (e.g., 123), then through at least one pass of the second heat-exchanger stage (e.g., 122), then through at least one pass of the first heat-exchanger stage (e.g., 121), then through at least one pass of the second heat-exchanger stage (e.g., 122), and then through at least one pass of the first heat-exchanger stage (e.g., 121). In the specific embodiment illustrated, for example, first heat-exchanger stage 121, second heat-exchanger stage 122, and third heat-exchanger stage 123 are arranged in apparatus 100 in an order such that the second fluid (e.g., indoor air 210) passes first through pass 576 of third heat-exchanger stage 123, then through pass 575 of second heat-exchanger stage 122, then through pass 573 of first heat-exchanger stage 121, then through pass 574 of second heat-exchanger stage 122, and then through pass 572 and pass 571 of first heat-exchanger stage 121.

In various embodiments, the first heat-exchanger stage (e.g., 121) has a first number of tubes, the second heat-exchanger stage (e.g., 122) has a second number of tubes, and the third heat-exchanger stage (e.g., 123) has a third number of tubes. In some embodiments, the first number of tubes may be equal to or less than (e.g., one less than) the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, for example. In the embodiment illustrated, for example, the first number of tubes and the second number of tubes are both five, and the third number of tubes is 36, which is substantially larger than the second number of tubes. As mentioned, other embodiments may have different numbers of tubes. For example, the third number of tubes (e.g., of third heat-exchanger stage 123) may be 10, 12, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 38, 40, 42, 44, 46, 48, or 50, as examples.

Various embodiments of the invention are, or include, methods, such as methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, such as those described herein, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel. Such methods may include, for example, (i.e., in any order, unless a particular order is required or indicated) at least certain acts.

FIG. 7 illustrates an example of such a method, method 700. In the embodiment shown, method 700 includes act 731 of forming or obtaining first-stage tubes (e.g., tubes 131 described herein). In particular embodiments, each first-stage tube (e.g., 131) may have a first diameter (e.g., 1.75 inches), a first 180 degree bend (e.g., 141), a second 180 degree bend (e.g., 142), or a combination thereof, for example. Another act found in various embodiments, and illustrated in FIG. 7, is act 732 of forming or obtaining second-stage tubes (e.g., 132). In certain embodiments, each second-stage tube (e.g., 132) may have a second diameter (e.g., 1.0 inches), a third 180 degree bend (e.g., 143), or both, for instance. In some embodiments, acts 731, 732, or both, may include obtaining tubing of the desired diameter and material, cutting the tubing to the desired lengths, and bending the tubing into the desired shapes (e.g., forming bend 141, 142, 143, or a combination thereof), for instance, using a tubing bender. In other embodiments, such tubing may be ordered and obtained already cut to length, already bent, or both, as further examples.

In the embodiment illustrated, method 700 also includes act 733 of forming or obtaining third-stage tubes (e.g., tubes 133 described herein). In some embodiments, act 733 may include obtaining tubing of the desired diameter and cutting the tubing to the desired lengths, for example. In other embodiments, the third-stage tubes (e.g., tubes 133 described herein) may be obtained already cut to length, may have fins (e.g., 333) already attached, or both, as other examples. Additionally, method 700 further includes, act 760 of forming or obtaining a junction plate (e.g., junction plate 160 described herein), act 761 of forming or obtaining a first collector (e.g., first collector 161 described herein), and act 762 of forming or obtaining second collector (e.g., second collector 162 described herein). These components may be made of sheet metal, such as steel, aluminized steel, or stainless steel, as examples, and acts 760, 761, and 762 may include cutting the sheet metal, stamping, bending, etc. In other embodiments, these components may be obtained precut, pre-bent, or both, as examples.

In the embodiment depicted, method 700 also includes act 721 of assembling a first heat-exchanger stage (e.g., stage 121 described herein). Act 721 may be accomplished using multiple (e.g., five) of the first-stage tubes (e.g., 131) formed in act 731, for example. In some such embodiments, the first heat-exchanger stage (e.g., 721, for instance, assembled in act 721) may include, for example, multiple passes, such as a first pass (e.g., pass 571 shown in FIG. 5), a second pass (e.g., pass 572), and a third pass (e.g., pass 573). Another act in method 700 is act 722 of assembling a second heat-exchanger stage (e.g., stage 122 described herein), for instance, using multiple (e.g., five) of the second-stage tubes (e.g., 132) formed in act 732, for example. In some such embodiments the second heat-exchanger stage (e.g., 121, for example, assembled in act 722) may include, for instance, one or more passes, such as a fourth pass (e.g., 574) and a fifth pass (e.g., 575), for example. In other embodiments, the first or second heat-exchanger stages (or both) may be obtained already formed, as other examples.

Yet another act in method 700, in the embodiment shown, is act 723 of forming or obtaining a third heat-exchanger stage (e.g., stage 123 described herein), for instance, using multiple (e.g., 36) of the third-stage tubes (e.g., 133) formed in act 733, for example. In some such embodiments the third heat-exchanger stage (e.g., 123, for example, formed or obtained in act 723) may include, for instance, one pass, such as a sixth pass (e.g., 576). In some embodiments, act 723 may include installing multiple fins (e.g., 333 shown in FIG. 3), which may be external to the third-stage tubes (e.g., 133), and may be bonded to the third-stage tubes (e.g., 133), for example, with solder or an interference fit, as examples. In some embodiments, for example, the third-stage tubes (e.g., 133) may be expanded to create an interference fit with holes in the fins (e.g., 333) by passing a bullet through the third-stage tubes (e.g., 133). In some embodiments, the third heat-exchanger stage (e.g., stage 123 described herein) may be obtained already assembled, as another example.

Further, method 700 includes act 755 of installing one or more burners (e.g., 155 shown in FIGS. 1 and 3). In some embodiments, for example, a burner (e.g., 155) may be installed (e.g., in act 755), at an entrance end (e.g., 151) of (e.g., each) first-stage tube (e.g., 131). Further still, method 700 also includes act 781 of connecting the second heat-exchanger stage (e.g., 122) to the first heat-exchanger stage (e.g., 121) (or vice versa). In some embodiments, the second heat-exchanger stage (e.g., 122) may be connected (e.g., in act 781) to the first heat-exchanger stage (e.g., 121), for instance, so that, when the furnace or unit (e.g., HVAC unit 250 shown in FIG. 2) is in operation, products of combustion from the burning of the fuel (e.g., at burner or burners 155) pass first through the first heat-exchanger stage (e.g., 121) and then through the second heat-exchanger stage (e.g., 122), for example, passing first through the first-stage tubes (e.g., 131) and then through the second-stage tubes (e.g., 132).

In some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act 731), forming or obtaining second-stage tubes (e.g., act 732), assembling the first heat-exchanger stage (e.g., act 721), and assembling the second heat-exchanger stage (e.g., act 722), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g., 131) and the second-stage tubes (e.g., 132), and assembling and arranging the first heat-exchanger stage (e.g., 121) and the second heat-exchanger stage (e.g., 122), for example, so that indoor air (e.g., 210 shown in FIG. 2) passing through the furnace (e.g., HVAC unit 250, for example, when the furnace or unit is in operation, for instance, when fan 255 is operating), passes (e.g., in predominant air flow direction 110) first through the fifth pass (e.g., pass 575 shown in FIG. 5), then through the third pass (e.g., pass 573), then through the fourth pass (e.g., pass 574), then through the second pass (e.g., pass 572), and then through the first pass (e.g., pass 571), for example.

Further, in various embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act 731), forming or obtaining second-stage tubes (e.g., act 732), assembling the first heat-exchanger stage (e.g., act 721), and assembling the second heat-exchanger stage (e.g., act 722), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g., 131) and the second-stage tubes (e.g., 132), and assembling and arranging the first heat-exchanger stage (e.g., 121) and the second heat-exchanger stage (e.g., 122) so that (e.g., when the furnace or unit, for example, 250, is in operation), the indoor air (e.g., 210) flows in a predominant flow direction (e.g., 110), for example, past the first-stage tubes (e.g., 131), past the second-stage tubes (e.g., 132), or both.

In some such embodiments, the first 180 degree bend (e.g., 141, which may have been formed or obtained in act 731, for example) may be oriented at a first angle (e.g., 501 shown in FIG. 5, for instance, from the predominant flow direction 110), and the absolute value of the first angle (e.g., 501) may be between 15 degrees and 75 degrees, for example. Similarly, in some embodiments, the second 180 degree bend (e.g., 142, which may have been formed or obtained in act 732, for example) may be oriented at a second angle (e.g., 502 from the predominant flow direction 110), and the absolute value of the second angle (e.g., 502) may (e.g., also) be between 15 degrees and 75 degrees, for instance. Further, in some embodiments, the third 180 degree bend (e.g., 143) may be oriented at a third angle (e.g., shown in FIG. 6, for instance, from the predominant flow direction 110), and the absolute value of the third angle (e.g., 603) may be less than 15 degrees, for example. In other embodiments, some or all of these angles (e.g., 501, 502, and 603) may fall within other ranges described herein, or may have values identified herein, as examples.

Moreover, in a number of embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act 731) and assembling the first heat-exchanger stage (e.g., act 721), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g., 131) and assembling and arranging the first heat-exchanger stage (e.g., 121) so that the first angle (e.g., 501) and the second angle (e.g., 502) have opposite signs (e.g., are in opposite directions from predominant flow direction 110).

Method 700 also includes, in the embodiment illustrated, act 782 of connecting the third heat-exchanger stage (e.g., 123) to the second heat-exchanger stage (e.g., 122), for instance, so that (e.g., when the furnace or unit, for example, HVAC unit 250, is in operation), products of combustion from burning of the fuel (e.g., at burners 155) pass first through the first heat-exchanger stage (e.g., 121), then through the second heat-exchanger stage (e.g., 122), and then through the third heat-exchanger stage (e.g., 123), for instance, passing first through the first-stage tubes (e.g., 131), then through the second-stage tubes (e.g., 132), and then through the third-stage tubes (e.g., 133).

Further, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act 731), forming or obtaining second-stage tubes (e.g., act 732), forming or obtaining the third heat-exchanger stage (e.g., act 723), assembling the first heat-exchanger stage (e.g., act 721), assembling the second heat-exchanger stage (e.g., act 722), and connecting the third heat-exchanger stage to the second heat-exchanger stage (e.g., act 782), for example, may further include, (or one or more other acts may include) for instance, assembling and arranging the first heat-exchanger stage (e.g., 121), the second heat-exchanger stage (e.g., 122), and the third heat-exchanger stage (e.g., 123) so that indoor air (e.g., 210) passing through the furnace (e.g., HVAC unit 250 shown in FIG. 2), for example, (e.g., when the furnace or unit 250, or fan 255, for instance, is in operation) passes (e.g., flowing upwards or in predominant airflow direction 110) first through the sixth pass (e.g., 576 shown in FIG. 5), then through the fifth pass (e.g., 575), then through the third pass (e.g., 573), then through the fourth pass (e.g., 574), then through the second pass (e.g., 572), and then through the first pass (e.g., 571), for example.

Further, in some embodiments, various acts, such as the acts of assembling the first heat-exchanger stage (e.g., act 721), assembling the second heat-exchanger stage (e.g., act 722), and connecting the third heat-exchanger stage to the second heat-exchanger stage (e.g., act 782), for example, may further include, for instance, (e.g., in various orders) terminating one end (e.g., end 351 shown in FIG. 3) of each of the first-stage tubes (e.g., 131) at the junction plate (e.g., 160), terminating two ends (e.g., 352) of each of the second-stage tubes (e.g., 132) at the junction plate (e.g., 160), terminating one end (e.g., end 453 shown in FIG. 4) of each of the third-stage tubes (e.g., 133) at the junction plate (e.g., 160), or a combination thereof, for instance. Some such embodiments may further include (e.g., within act 781), installing a first collector (e.g., 161 shown in FIG. 1) against the junction plate (e.g., 160) forming a first enclosed passageway, for example, that connects the first-stage tubes (e.g., 131) to the second-stage tubes (e.g., 132). Further, some embodiments may further include (e.g., within act 782), installing a second collector (e.g., 162 shown in FIG. 1) against the junction plate (e.g., 160) forming a second enclosed passageway, for example, that connects the third-stage tubes (e.g., 133) to the second-stage tubes (e.g., 132).

Furthermore, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act 731), forming or obtaining second-stage tubes (e.g., act 732), forming or obtaining the third-stage tubes (e.g., act 733) and forming or obtaining the third heat-exchanger stage (e.g., act 723), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g., 131) and the second-stage tubes (e.g., 132) and forming or obtaining the third heat-exchanger stage (e.g., 123) so that the third-stage tubes (e.g., 133) have a third diameter. In some embodiments, the first diameter (e.g., of first-stage tubes 131) may be substantially larger than the second diameter (e.g., of second-stage tubes 132), the second diameter (e.g., of second-stage tubes 132) may be substantially larger than the third diameter (e.g., of third-stage tubes 133), or both, as examples.

Moreover, in some embodiments, particular acts, such as the acts of assembling the first heat-exchanger stage (e.g., act 721), assembling the second heat-exchanger stage (e.g., act 722), and forming or obtaining the third heat-exchanger stage (e.g., act 723), for example, may include, for instance, assembling the first heat-exchanger stage (e.g., 121) and assembling the second heat-exchanger stage (e.g., 122) and forming or obtaining the third heat-exchanger stage (e.g., 123) so that the first heat-exchanger stage (e.g., 121) has a first number of tubes, the second heat-exchanger stage (e.g., 122) has a second number of tubes, and the third heat-exchanger stage (e.g., 123) has a third number of tubes. In certain embodiments, the first number of tubes (e.g., 5) may be equal to or one less than (or a different number less than) the second number of tubes (e.g., 5 or 6), the third number of tubes (e.g., 36) may be substantially larger than the second number of tubes, or both, as examples.

Other embodiments may be apparent to a person of ordinary skill in the art having studied this document, and may include features or limitations described herein, shown in the drawings, or both. Various methods may include part or all of the acts shown in FIG. 7, described herein, or known in the art, as examples. 

1. An apparatus for transferring heat, the apparatus comprising: a first heat-exchanger stage comprising multiple parallel first-stage tubes, each first-stage tube having a first 180 degree bend and a second 180 degree bend; and a second heat-exchanger stage comprising multiple parallel second-stage tubes, each second-stage tube having a third 180 degree bend; wherein: the first-stage tubes and the second-stage tubes are configured to contain within a flowing first fluid; the first-stage tubes and the second-stage tubes are configured to transfer heat between the first fluid and a second fluid external to the first-stage tubes and external to the second-stage tubes, wherein, when the apparatus is in operation, the second fluid flows in a predominant flow direction past the first-stage tubes and past the second-stage tubes; the first 180 degree bend is oriented at a first angle from the predominant flow direction, wherein an absolute value of the first angle is between 15 degrees and 75 degrees; the second 180 degree bend is oriented at a second angle from the predominant flow direction, wherein an absolute value of the second angle is between 15 degrees and 75 degrees; and the third 180 degree bend is oriented at a third angle from the predominant flow direction, wherein an absolute value of the third angle is less than 15 degrees.
 2. The apparatus of claim 1 wherein the first angle and the second angle have opposite signs.
 3. The apparatus of claim 1 further comprising a third heat-exchanger stage comprising multiple parallel third-stage tubes.
 4. The apparatus of claim 3 wherein the first heat-exchanger stage has three passes through the second fluid, the second heat-exchanger stage has two passes through the second fluid, and the third heat-exchanger stage has one pass through the second fluid.
 5. The apparatus of claim 3 wherein the third heat-exchanger stage comprises multiple fins external to the third-stage tubes; wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged in an order such that the second fluid passes first through the third heat-exchanger stage and then through at least one pass of the second heat-exchanger stage, and such that the second fluid passes through at least one pass of the first heat-exchanger stage last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage; and wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage.
 6. The apparatus of claim 3 wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged in an order such that the second fluid passes first through the third heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage.
 7. The apparatus of claim 3 wherein the first-stage tubes have a first diameter, the second-stage tubes have a second diameter, the third-stage tubes have a third diameter, the first diameter is substantially larger than the second diameter, and the second diameter is substantially larger than the third diameter.
 8. The apparatus of claim 3 wherein the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, the third heat-exchanger stage has a third number of tubes, the first number of tubes is equal to or one less than the second number of tubes, and the third number of tubes is substantially larger than the second number of tubes.
 9. The apparatus of claim 3 further comprising a junction plate wherein one end of each of the first-stage tubes terminates at the junction plate, two ends of each of the second-stage tubes terminate at the junction plate, and one end of each of the third-stage tubes terminates at the junction plate, wherein the apparatus further comprises a first collector that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes, and a second collector that seals against the junction plate and forms a second enclosed passageway that connects the second-stage tubes to the third-stage tubes.
 10. The apparatus of claim 1 wherein: the absolute value of the first angle is between 30 degrees and 60 degrees; the absolute value of the second angle is between 30 degrees and 60 degrees; and the absolute value of the third angle is less than 10 degrees.
 11. The apparatus of claim 1 wherein: the absolute value of the first angle is between 40 degrees and 50 degrees; the absolute value of the second angle is between 40 degrees and 50 degrees; and the absolute value of the third angle is less than 5 degrees.
 12. The apparatus of claim 1 wherein: the first heat-exchanger stage and the second heat-exchanger stage are arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage and then through the second heat-exchanger stage; and the first heat-exchanger stage and the second heat-exchanger stage are arranged in an order such that the second fluid passes first through one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage.
 13. The apparatus of claim 1 wherein the first-stage tubes have a first diameter, the second-stage tubes have a second diameter, and the first diameter is substantially larger than the second diameter; the apparatus further comprising a junction plate, wherein one end of each of the first-stage tubes terminates at the junction plate and at least one end of each of the second-stage tubes terminates at the junction plate, wherein the apparatus further comprises a first collector that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes.
 14. The apparatus of claim 1 further comprising multiple burners wherein each of the first-stage tubes has an entrance end where air enters the first-stage tube, and wherein each burner is located at the entrance end to one of the first-stage tubes and each burner is configured to burn a fuel in one of the first-stage tubes, wherein combustion gasses from the air and the fuel form the first fluid.
 15. The apparatus of claim 1 wherein the apparatus is a furnace.
 16. The apparatus of claim 1 wherein the apparatus is an HVAC unit.
 17. The apparatus of claim 1 wherein the apparatus is an HVAC system.
 18. The apparatus of claim 1 wherein the apparatus is a building having an HVAC system.
 19. An HVAC unit comprising a furnace for heating indoor air that passes through the HVAC unit, the furnace comprising: a first heat-exchanger stage comprising multiple parallel first-stage tubes; a second heat-exchanger stage comprising multiple parallel second-stage tubes; a third heat-exchanger stage comprising multiple parallel third-stage tubes; at least one burner configured to burn a fuel in air forming combustion gasses; a junction plate wherein an exit end of each of the first-stage tubes terminates at the junction plate, two ends of each of the second-stage tubes terminate at the junction plate, and one end of each of the third-stage tubes terminates at the junction plate; a first collector that seals against the junction plate forming a first enclosed passageway that transfers the combustion gasses from the first-stage tubes to the second-stage tubes; and a second collector that seals against the junction plate forming a second enclosed passageway that transfers the combustion gasses from the second-stage tubes to the third-stage tubes; and wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged and connected in an order such that the combustion gasses pass first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage.
 20. The HVAC unit of claim 19 wherein each first-stage tube has a first 180 degree bend and a second 180 degree bend, and each second-stage tube has a third 180 degree bend.
 21. The HVAC unit of claim 20 wherein, when the HVAC unit is in operation, the indoor air flows in a predominant flow direction past the first-stage tubes, past the second-stage tubes, and past the third-stage tubes; the first 180 degree bend is oriented at a first angle from the predominant flow direction, wherein an absolute value of the first angle is between 15 degrees and 75 degrees; the second 180 degree bend is oriented at a second angle from the predominant flow direction, wherein an absolute value of the second angle is between 15 degrees and 75 degrees; and the third 180 degree bend is oriented at a third angle from the predominant flow direction, wherein an absolute value of the third angle is less than 15 degrees.
 22. The HVAC unit of claim 19 wherein the third heat-exchanger stage comprises multiple fins external to the third-stage tubes; wherein the first heat-exchanger stage has three passes through the indoor air, the second heat-exchanger stage has two passes through the indoor air, and the third heat-exchanger stage has one pass through the indoor air; and wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged in an order such that the indoor air passes first through the third heat-exchanger stage and then through at least one pass of the second heat-exchanger stage, and such that the indoor air passes through at least one pass of the first heat-exchanger stage last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage.
 23. The HVAC unit of claim 19 wherein the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage are arranged in an order such that the indoor air passes first through the third heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage.
 24. The HVAC unit of claim 19 wherein the first-stage tubes have a first diameter, the second-stage tubes have a second diameter, the third-stage tubes have a-third diameter, the first diameter is substantially larger than the second diameter, and the second diameter is substantially larger than the third diameter; and wherein the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, the third heat-exchanger stage has a third number of tubes, the first number of tubes is equal to the second number of tubes, and the third number of tubes is substantially larger than the second number of tubes.
 25. A method of making a compact and efficient furnace to heat indoor air by burning a fuel, the method comprising in any order at least the acts of: forming or obtaining first-stage tubes, each first-stage tube having a first diameter, a first 180 degree bend and a second 180 degree bend; and forming or obtaining second-stage tubes, each second-stage tube having a second diameter and a third 180 degree bend; assembling a first heat-exchanger stage using multiple of the first-stage tubes, wherein the first heat-exchanger stage comprises a first pass, a second pass, and a third pass; assembling a second heat-exchanger stage using multiple of the second-stage tubes, wherein the second heat-exchanger stage comprises a fourth pass and a fifth pass; installing a burner at an entrance end of each first-stage tube; and connecting the second heat-exchanger stage to the first heat-exchanger stage so that, when the furnace is in operation, products of combustion from burning of the fuel at the burner pass first through the first heat-exchanger stage and then through the second heat-exchanger stage, passing first through the first-stage tubes and then through the second-stage tubes; wherein the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage, and assembling the second heat-exchanger stage comprise forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage so that indoor air passing through the furnace, when the furnace is in operation, passes first through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass.
 26. The method of claim 25 wherein the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage and assembling the second heat-exchanger stage comprise forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage so that: when the furnace is in operation, the indoor air flows in a predominant flow direction past the first-stage tubes and past the second-stage tubes; the first 180 degree bend is oriented at a first angle from the predominant flow direction, wherein an absolute value of the first angle is between 15 degrees and 75 degrees; the second 180 degree bend is oriented at a second angle from the predominant flow direction, wherein an absolute value of the second angle is between 15 degrees and 75 degrees; and the third 180 degree bend is oriented at a third angle from the predominant flow direction, wherein an absolute value of the third angle is less than 15 degrees.
 27. The method of claim 26 wherein the acts of forming or obtaining first-stage tubes, and assembling the first heat-exchanger stage comprise forming or obtaining the first-stage tubes and assembling and arranging the first heat-exchanger stage so that the first angle and the second angle have opposite signs.
 28. The method of claim 25 further comprising an act of forming or obtaining a third heat-exchanger stage having multiple third-stage tubes, wherein the third heat-exchanger stage comprises a sixth pass and the third heat-exchanger stage has multiple fins external to the third-stage tubes.
 29. The method of claim 28 further comprising an act of: connecting the third heat-exchanger stage to the second heat-exchanger stage so that, when the furnace is in operation, products of combustion from burning of the fuel at the burner pass first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage, passing first through the first-stage tubes, then through the second-stage tubes, and then through the third-stage tubes; wherein the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, forming or obtaining the third heat-exchanger stage, assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage further comprise assembling and arranging the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage so that indoor air passing through the furnace, when the furnace is in operation, passes first through the sixth pass, then through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass.
 30. The method of claim 29 further comprising, in any order, the acts of: forming or obtaining a junction plate; forming or obtaining a first collector; and forming or obtaining second collector; and wherein the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage further comprise, in any order, the acts of: terminating one end of each of the first-stage tubes at the junction plate, terminating two ends of each of the second-stage tubes at the junction plate, terminating one end of each of the third-stage tubes at the junction plate, installing the first collector against the junction plate forming a first enclosed passageway that connects the first-stage tubes to the second-stage tubes, and installing the second collector against the junction plate forming a second enclosed passageway that connects the second-stage tubes to the third-stage tubes.
 31. The method of claim 28 wherein the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, and forming or obtaining the third heat-exchanger stage comprise forming or obtaining the first-stage tubes and the second-stage tubes and forming or obtaining the third heat-exchanger stage so that the third-stage tubes have a third diameter, the first diameter is substantially larger than the second diameter, and the second diameter is substantially larger than the third diameter; and wherein the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage comprise assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage so that the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, the third heat-exchanger stage has a third number of tubes, the first number of tubes is equal to or one less than the second number of tubes, and the third number of tubes is substantially larger than the second number of tubes. 